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That’s because it wasn’t until 2000 that the Census Bureau allowed Americans to choose more than one racial category to describe themselves. Before that, you could pick only one, and people with mixed backgrounds often struggled over the decision about which box to check.

When the Census Bureau made that change, it had an especially profound impact in Seattle. That’s because even though Seattle ranks only 15th in size among U.S. metropolitan areas, our population of multiracial people — about 233,000 — is the fourth-largest. New York, Los Angeles and San Francisco are the top three, in order.

And among the 50 largest metros, we have the second-highest percentage of people who identify as two or more races. At 6.4 percent, it’s more than twice the U.S. average, according to census data.

The only major metro with a higher percentage than Seattle is Oklahoma City, at 6.7 percent. But the composition of that area’s multiracial population is very different from Seattle’s.

In Oklahoma City, people who are both white and Native American account for more than half of those who say they have at least two races in their background.

But in Seattle, there is no single combination that makes up the majority of multiracial people. The largest group here is people claiming both white and Asian ancestry — about one-third.

The relatively recent acknowledgment of a multiracial identity represents just one chapter in our constantly evolving understanding and attitudes about race. While the census has included questions about race since its inception in 1790, the nature of those questions has changed dramatically.

And through 1960, census-takers themselves determined the race of the individuals they counted, reflecting the perception that race was a fixed physical trait. It is only since 1970 that race became a matter of self-identity on census forms.

And for multiracial people in particular, that identity can be fluid. Whether someone with a mixed-race background identifies as such is highly personal. And data from the Pew Research Center show that people with certain racial backgrounds are more likely than others to identify as multiracial.

About 70 percent of adults who are both white and Asian identify as biracial — more than any other combination. In contrast, only about a quarter of adults who are white and Native American consider themselves biracial.

So the fact that Seattle has a high percentage of Asians could partly explain why the number of multiracial people here is so high — a higher percentage chooses to identify as such.

Pew’s research shows that, overall, the number of Americans with mixed-race backgrounds could be as much as three times higher than the number who identify as multiracial.

To further complicate the issue, the Census Bureau considers Hispanic an ethnicity rather than a racial category — even though Americans tend to think of Hispanic as a race.